You can see why a prosecutor might think that the young woman and her boyfriend might be involved in the case, and why he might want to find evidence that supported his view. You can see why he might, in the glare of the world's media, feel under pressure to solve the case as quickly as he could. But it's quite hard to see why he would stick to that case, even when the evidence for it seemed to be extremely flimsy, and why he never seemed to consider the possibility that that lack of evidence might mean he was wrong. Unless, that is, it was more important for him to be seen to be right than to solve the crime.
So where exactly does Mignini's demand that Knox be subjected to SIX MONTHS OF SOLITARY CONFINEMENT fit into this crazy "saving face" cover story?